Infrared imaging sensor

ABSTRACT

A system for detection and control of deposition on pendant tubes in recovery and power boilers includes one or more deposit monitoring sensors operating in infrared regions of about 4 or 8.7 microns and directly producing images of the interior of the boiler, or producing feeding signals to a data processing system for information to enable a distributed control system by which the boilers are operated to operate said boilers more efficiently. The data processing system includes an image pre-processing circuit in which a 2-D image formed by the video data input is captured, and includes a low pass filter for performing noise filtering of said video input. It also includes an image compensation system for array compensation to correct for pixel variation and dead cells, etc., and for correcting geometric distortion. An image segmentation module receives a cleaned image from the image pre-processing circuit for separating the image of the recovery boiler interior into background, pendant tubes, and deposition. It also accomplishes thresholding/clustering on gray scale/texture and makes morphological transforms to smooth regions, and identifies regions by connected components. An image-understanding unit receives a segmented image sent from the image segmentation module and matches derived regions to a 3-D model of said boiler. It derives a 3-D structure the deposition on pendant tubes in the boiler and provides the information about deposits to the plant distributed control system for more efficient operation of the plant pendant tube cleaning and operating systems.

This application is a Continuation-in-Part, and claims the benefit of,U.S. patent application Ser. No. 11/137,089 filed on May 24, 2005, whichapplication issued as U.S. Pat. No. 7,437,025 on Oct. 14, 2008, andwhich was a divisional application of, and claimed the benefit of, U.S.application Ser. No. 10/168,277 filed on Jun. 14, 2002 which issued onJun. 21, 2005 as U.S. Pat. No. 6,909,816.

This invention pertains to an infrared sensing for inspecting equipmentcondition and operation in the interior of high temperature processequipment, and more particularly to a mid-band infra-red imaging systemthat is tuned to a particular spectrum of infra-red radiation to seeinto the otherwise visually impenetrable interior of high temperatureprocess equipment.

BACKGROUND OF THE INVENTION

In 1995, about 82% of the wood pulp consumed at US paper and paperboardplants was produced using the Kraft process. Although the proportion ofpulp from this source is likely to decline as new processes come online, it is expected that well over 50% of wood pulp production willstill be produced in 2020 using the Kraft process.

In the Kraft pulp production process, a fibrous material, most commonlywood chips, are broken down into pulp in a digester under pressure in asteam-heated aqueous solution of sodium hydroxide and sodium sulfide,called white liquor. After cooking in the digester, the pulp isseparated from the residual liquid called black liquor. Black liquor isan aqueous solution containing wood lignins, organic material, andinorganic compounds oxidized in the digester during the cooking process.It is concentrated and then burned in a recovery boiler to generatesteam, which is used in the pulp mill for pulp cooking and drying, andother energy requirements. The material remaining after combustion ofthe black liquor, called smelt, is collected in a molten bed at thebottom of the boiler and discharged to a dissolving tank to be recycledinto new white liquor.

Kraft chemical and energy recovery boilers, in which the black liquor isburned, are large and expensive, with capacities installed in the last30 years for pulp mills typically exceeding 1000 tons of pulp per day.It is difficult economically to add small incremental units of boilercapacity, so the capacity of the chemical recovery boiler is often thefactor limiting the capacity of the entire pulp mill.

The effective burning capacity of recover boilers is frequentlydetermined by the processes governing the deposition of fume,intermediate sized particles, and carryover of partially burntliquor/smelt drops on heat transfer surfaces of the steam and watertubes in the boiler, and the attendant plugging of gas passages betweenand around those pendant steam and water tubes. Much effort has beenmade and continues to be made to improving the understanding of themechanism of particulate and vapor deposition on the tubes. However,there are still no reliable on-line methods for systematically detectingthe presence and build-up rates of these deposits.

Various efforts to control the rate and quantity of deposits on thependant tubes in the boiler have been undertaken in the past. Theseinclude adjustments to conditions of combustion, such as the nozzlesthat spray the black liquor into the combustion chamber, and the way airis introduced into the combustion chamber. They also include systems,such as soot blowers, for removing deposits on the tubes before theyseriously impact the operation of the boiler. These control efforts aremost effective when they are immediately correlated to the results theyare intended to produce, but heretofore there has been no reliablemethod of determining directly the amount of deposits on the pendanttubes. Such control efforts have therefore necessarily been based onindirect measurements and considerations, and have usually yieldedunsatisfactory results.

The severe environment of boilers, namely the high temperature,turbulent gas flow, particle laden atmosphere, and intensity ofradiation have made it difficult to develop a sensing system fordetection and control of deposition on pendant tubes in Kraft recoveryboilers that would be economically viable as a commercial product.Attempts to use near-IR cameras for direct monitoring of pendant tubedeposits have failed to reliably produce good images over the span oflarge boilers, and devices operating at longer wavelengths have beenimpractical for boiler-side use because of prohibitive expense and theneed for reliable cryogenic cooling.

U.S. Pat. No. 4,539,588 entitled “Imaging of Hot Infrared EmittingSurfaces Obscured by Particulate Fume and Hot Gasses” issued on Sep. 3,1985 to Peter C. Ariessohn and R. K. James discloses an improvement inthe technology of the time, but operated in a wavelength region of1.5-1.8 micron, which has a relatively high susceptibility to lightscattering by particles in the boiler gas stream.

Thus, there has long been a serious need for a deposition detectionsystem for recovery boiler pendant tubes to solve the unfulfilledrequirement to monitor the degree and distribution of fume, intermediatesized particles, and carryover particle depositions on recovery boilertubes.

In addition to the need for a deposition detection system for directmonitoring of pendant tube deposits, there has been a long standing needfor inspection equipment that would reveal important information aboutthe condition and operation in the interior of equipment in many hightemperature process installation such as furnaces, boilers, gasifiers,process heaters, hot gas filtration systems, and ash hoppers, and alsoof equipment that operates at intermediate temperatures in the region ofabout 500° F.-800° F. such as selective catalytic reduction chambers,ducts, electrostatic precipitators.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a method and apparatus for directlymonitoring the depositions on recovery boiler pendant tubes, and forinspecting the interior of high and intermediate temperature processequipment to reveal important information about its condition andoperation.

The invention includes a focal plane array camera capable of creatingimages in a particular range of infrared radiation that has lowabsorption by molecules in the gas stream in a chemical recovery boileror other high temperature process equipment, and is not scatteredsignificantly by particles normally present in the gas in the processequipment. Another aspect of the invention is a system of one or moredeposit monitoring sensors feeding signals to a data processing systemunder control of a distributed control system. Preferably, the depositmonitoring sensors include focal plane array cameras operating in themid-infra-red band, in the region of about 4-12 microns wavelength.Clear images can be obtained at a low cost of the boiler interior andparticularly of the pendant water and steam tubes in the boiler toenable for the first time a visual real time inspection of the conditionof the tubes and depositions thereon so that control schemes can beimplemented.

DESCRIPTION OF THE DRAWINGS

The invention and its many attendant features and advantages will becomeclear upon reading the following detailed description of the preferredembodiments, in conjunction with the following drawings, wherein:

FIG. 1 is a schematic diagram of the invention installed in a Kraftrecovery boiler;

FIG. 2 is a schematic elevation of a monitoring sensor shown in FIG. 1;

FIG. 3 is an elevation of a hand-held sensor in accordance with thisinvention;

FIG. 4 is an elevation of the hand-held sensor of FIG. 3 showing the airflow system for the lens tube;

FIG. 5 is a sectional end elevation of the a monitoring sensors shown inFIG. 2;

FIG. 6 is a sectional elevation of the a monitoring sensor shown in FIG.3 along lines 6-6 in FIG. 4;

FIG. 7 is an enlarged sectional elevation of the distal end of the amonitoring sensor shown in FIG. 6;

FIG. 8 is a schematic diagram of the optical elements in the sensorshown in FIG. 3;

FIG. 9 is a graph showing the light transmission over a range ofwavelengths in a recovery boiler; and

FIG. 10 is a schematic flow diagram of the process of receiving datafrom the monitoring sensors in FIGS. 1 and 3 to data input to thedistributed control system in FIG. 1 for control of deposition controlsystems in the boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, and more particularly to FIG. 1 thereof, adeposition detection system in accordance with this invention is showninstalled in a Kraft recovery boiler 30. The deposition detection systemincludes one or several monitoring sensors 35, described in detailbelow, which acquire data in the midband infra-red spectrum within aparticular field of view from the interior of the recovery boiler. Themonitoring sensors 35 could be fixed in position to permanently monitorparticular areas within the boiler, or could be integrated hand-heldunits 36 shown in FIG. 3.

The sensor 35, shown in more detail in FIGS. 2 and 5-7, converts theacquired infrared data to electrical signals, which are conducted viaelectrical lines 37 to a sensor data processing system 40, shown in moredetail in FIG. 10 and described in detail below. The hand-held sensor 36shown in FIG. 3 converts the acquired infrared data directly to an imagethat is viewed on a display inside a hood 42 at the viewer end of acamera body 44.

A distributed control system 45, which is the computer system used bypaper mill or power plant operators for controlling the operation of theplant, is connected to the sensor data processing system 40 by a link 47for advanced control of the boiler operations in accordance with theinformation supplied by the sensor data processing system 40, withoperator judgement and analysis where necessary, to minimize depositionof the pendant steam tubes 49 and otherwise maximize plant efficiency.

Turning now to FIG. 2, one embodiment of the deposit monitoring sensor35 is shown having a focal plane array camera 50 and a lens tube 55connected to the camera 50 by way of a spectral band-pass filter 60 thatlimits the light admitted into the camera 50 to a particular band in themid-IR spectrum.

The imaging optics consist of the double-layered steel lens tube 55,shown in FIGS. 5-7, containing a train of ZnSe or, amorphous siliconlenses 65. The front lens 70 serves as the objective, and has a field ofview of at least 20°. Three other lenses 75, 76 and 77 serve as transferoptics, transporting the image formed by the objective onto the focalplane array 78 of the camera 50. The electrical signals from the imagingarray 78 are processed in the electronic circuitry 79 and transmitted toa remote processing system, in the case of the monitoring system shownin FIG. 2, or displayed on a display such as an LCD display screen 80 inthe case of the hand-held unit shown in FIG. 3. The total length of thelens tube is about 36 inches, permitting the focal plane to be locatedremotely from the boiler port. The lens tube 55 is cooled and purged bya constant stream of air supplied at about 30 psi through a gas coupling81 into the space between the inner and outer tubes of thedouble-layered lens tube 55, through which the air flows and exits outthrough an axial opening 85 at the distal end of the lens tube 65.

Several camera models could be used: a ferroelectric array camera, aPtSi camera, and a Si microbolometer array camera. Also, an InSb arraycamera operating in the 3.9 micron wavelength region, has producedadequate images but was determined to be impractical because of its costand the limited lifetime of the necessary low temperature coolingsystems required for operation of the camera. The ferroelectric arraycamera is attractive because it does not require cryogenic cooling, doesnot require frequent calibration and is relatively inexpensive. However,it does use a semi-transparent “choppers” wheel to limit the intensityof the light to the array. The chopper wheel introduces its own set ofproblems such as the superimposition of artifacts such as curved linesacross the image. These problems can be addressed by changing requiredsolutions to achieve satisfactory images. The PtSi array camera requirescryogenic cooling and is quite expensive, making it a less preferredversion of the usable cameras. The microbolometer array camera does notrequire cryogenic cooling and does not use a chopper. It also hassignificantly greater dynamic range than the ferroelectric array camera.However, it may require frequent (once-a-day) re-calibration to produceacceptable images, and is significantly more expensive than theferroelectric array camera.

The preferred camera is a ferroelectric array camera modified to viewinfrared radiation in a wavelength band of about 3.5-4.0 microns,preferably about 3.9 microns; or infrared radiation in a wavelength bandof about 8.5-9.0 microns, preferably about 8.7 microns. This cameraproduces clear images in the system outlined above and is inexpensiveenough to be affordable for pulp mills to purchase and use. Weanticipate that other imaging arrays usable in our camera will bedeveloped that will be usable in the system shown in FIG. 1

The sensor shown in FIG. 3 includes the camera body 44 connected to thelens tube 65 by way of an intermediate structure 85. The intermediatestructure 85 includes an adjustable iris 90 and the lens 60, which isaxially movable to give the lens train the ability of focus in alow-light, wide aperture condition. The hand-held unit 37 has a powerswitch 94 and an electrical connector 96, which provides the ability toconnect electronically into the distributed control system 45. The lenstube 55 is connected to a source 98 of air pressure through a pressureregulator 97 and a flexible air hose 99.

The graph on FIG. 9 illustrates the benefits of operating in the regionsof about 4 and 8.7 microns. As illustrated, there are several “windows”available to viewing the interior of a chemical recovery boiler byvirtue of the light absorption characteristics of the gas and vapors inthe gas stream of a chemical recovery boiler for a pulp mill. Thevisibility of the boiler interior at these wavelengths is alsoinfluenced by the scattering effect of the particles in the boiler gas.The effectiveness of this particle scattering is greatly decreased atlonger wavelengths, and for wavelengths in excess of 3 microns does notsignificantly degrade images of recovery boiler interiors in the upperfurnace and convection-pass sections. By operating an a region of lowabsorption and low scatter of the gas molecules and particles,respectively, in the boiler gas stream, the resolution of the imagesthat are possible by infrared imaging in the chemical recovery boiler ismaximized.

Turning now to FIG. 10, an image processing system 40 and a link to oneversion of the distributed control system is shown having a video input37 from the camera 50 to an image pre-processing circuit 95 in which the2-D image is captured and noise filtering is performed in a low passfilter. Array compensation is accomplished to correct for pixelvariation and dead cells, etc., and geometric distortion is corrected byimage system compensation. A cleaned image 100 is sent from the imagepre-processing circuit to an image segmentation module 105 where theimage of the recovery boiler interior is separated into background,pendant tubes, and deposition. Thresholding/clustering on grayscale/texture is accomplished and morphological transforms to smoothregions are made. Regions are identified by connected components. Thesegmented image 110 is sent from the image segmentation module 105 to animage-understanding unit 115 where derived regions are matched to a 3-Dmodel of the recovery boiler and a 3-D structure 120 of the depositionis inferred. Those deposition estimates can be provided to thedistributed control system to update the computer model and state 125 ofthe recovery boiler which is fed back in a closed loop to continuallyupdate the image understanding unit 115. The deposition estimates 120are fed to the “soot-blower” control 130 for optimized control of thesteam cleaning system for the pendant tubes 49 in the boiler.

A control scheme is envisioned that utilizes the information from thedeposition detection system to control or minimize further deposition,or optimize deposit removal processes. From the processed images, thesystem identifies the location of deposits and activates the steamcleaners, or “soot-blowers”, that are most appropriate to clean theaffected location and prevent pluggage. Currently, the “soot-blowers”are operated “blind” on a timed cycle. Operating the soot-blowers onlywhere and when there are deposits needing removal will minimize thesteam usage as well as tube wear caused by unnecessary over-cleaning.Moreover, it is now possible for the first time to accurately relate thedeposition rate to the liquor burning parameters, so the boileroperation can be optimized to minimize deposits on the pendant tubes.

The method and apparatus of this invention also allows for inspectingthe interior of high temperature process equipment to reveal importantinformation about its condition and operation. Such equipment includesfurnaces, boilers, gasifiers, process heaters, ducts, hot gas filtrationsystems, electrostatic precipitators, and ash hoppers. It utilizes“windows” between molecular absorption bands in most combustion andgasification processes. These windows occur in the region of 1.25microns, 1.65 microns, 2.2 microns, 3.9 microns, and 8.7 microns,although for purposes of this invention, the windows in the region of3.9 and 8.7 microns are of the greatest interest. At a given pressure,these windows get a bit wider and clearer as temperature increases dueto the decrease in gas density, so within a given window, visibilityimproves as temperature increases. The following table shows the extentof these windows at different temperatures for several fuels. The tablereflects only the molecular absorption part of the picture and particlescattering needs to be considered separately, as discussed in moredetail below.

For the combustion cases (black liquor, coal, fuel oil, and naturalgas), the window limits are set as the wavelengths where the opticaltransmission over a 50 foot path=90%. So, within the window, opticaltransmission is >90%. The 8 to 12 micron windows—which contain a largenumber of weak absorption lines—are an exception to this rule. For thatcase, the window limits are set to be an optical transmission level of60%, where a majority of the region in between is above that level, butnot all. Therefore, there will be more interference and poorervisibility within the 8-12 micron window than with any of the otherwindows. For coal gasification, the pressure is assumed to be 400 psiand the path length to be 6 feet (typical for commercial gasifiers).Otherwise, the same comments apply as in the combustion cases and thewindows with good visibility turn out to be very similar to those forthe combustion cases.

1.2 μm 1.65 μm 2.2 μm 3.8 μm 8-12 μm window window window window windowTemp. λ min λ max λ min λ max λ min λ max λ min λ max λ min λ max Deg F.(μm) (μm) (μm) (μm) (μm) (μm) (μm) (μm) (μm) (μm) Black Liquor 500 1.171.33 1.5 1.72 2.1 2.3 3.5 4.05 8.5 12.2 1500 1.17 1.33 1.5 1.75 2.05 2.33.47 4.15 8.4 12.2 2500 1.15 1.33 1.45 1.77 1.98 2.46 3.36 4.15 8.2312.2 Coal 500 1.16 1.33 1.49 1.75 2.09 2.37 3.42 3.97 9.28 12.16 15001.16 1.33 1.49 1.75 2.04 2.39 3.42 4.15 8.53 12.16 2500 1.15 1.33 1.441.77 1.98 2.46 3.12 4.15 8.23 12.2 Fuel Oil 500 1.16 1.33 1.49 1.77 2.092.39 3.42 4.15 8.53 12.16 1500 <1 1.33 1.46 1.77 2.04 2.41 3.42 4.158.53 12.22 2500 <1 1.34 1.44 1.77 1.97 2.46 3.09 4.15 8.11 12.28 NaturalGas 500 1.17 1.32 1.5 1.75 2.07 2.32 3.43 4.15 8.53 12.22 1500 1.15 1.321.5 1.76 2.03 2.35 3.43 4.15 8.53 12.22 2500 <1 1.33 1.44 1.77 1.97 2.473.11 4.15 8.23 12.22 Coal Gasifier 2500 1.17 1.33 11.5 1.76 2.03 2.283.48 4.15 8.23 12.22

The figures for the preferred 3.8 micron window indicate that it affordsnearly 100% transmission throughout the window. The other figures shownin bold type offer good transmissivity although the spectral band is agood deal narrower, except the two cases where the 8-12 micron window isfairly clear at high temperatures where the gas density is low.

In systems with “dirty” fuels (coal, oil, and black liquor) there willbe high particle loading and that will affect visibility. In recoveryboilers the particles reduce the desirability of the 1.2 micron window,but the other windows give good results since the scattering efficiencyof the sub-micron fume particles decreases very rapidly with increasingwavelength. In coal combustion systems, the particles are much bigger—onthe order of 5 to 50 microns, so they interfere with visibility in allthe windows. The particles in the case of fuel oil combustion are fairlysmall (on the order of a few microns in diameter, or less) and therewill be fewer of them, so visibility should be good in all the windowsshown except the 8-12 micron window at low temperatures (below 2000°F.). Natural gas should produce no particles unless very fuel richconditions lead to soot formation. In that case, only molecularabsorption will play a role.

Convertible (Dual Use) Mounting Arrangement

A portable inspection camera may be removably mounted in a fixed mounthousing to facilitate use in continuous process monitoring. The fixedmount housing includes a permanently attached lens tube and filter. Thehousing is rigidly mounted on the outer wall of the process vessel orcan be mounted on an automatic retraction mechanism that allows thecamera and lens tube to be retracted from the process by remote control.

For portable inspection applications when the camera is dismounted fromthe fixed mount housing, a short (18 to 24 inch long) detachable lenstube is connected to the body of the inspection camera by a bayonetmount, which facilitates easy mounting and dismounting from the camerabody. This lens tube contains a bandpass filter, so when the lens isremoved, a different lens/filter combination can be attached to thecamera to allow the system to operate in a different spectral window.The housing also contains a vortex cooler to maintain the camera at anacceptable temperature for reliable operation.

In a recovery boiler application, for instance, the permanent mountarrangement would employ a glass lens tube and a narrow bandpass filteroperating in the 1.65 micron window to allow the camera to view thesmelt bed region which is normally at 2200 to 2500° F. The camera canthen be removed from the housing and a lens tube employing silicon,germanium or zinc selenide lenses (or lenses made of any other materialwhich is transparent at 4 microns) and also containing a bandpassfilter—which limits the sensitivity of the system to radiation in the3.5 to 4 micron range—would be attached to the camera. A handle and aheat shield would also be attached to convert the camera into a portableinspection camera suitable for viewing deposits on tubes in theconvective sections of the boiler which operate at temperatures in therange ˜1200 to 2000° F. The use of the 3.8 micron spectral band takesadvantage of the fact that the black body emission peak occurs near thatwavelength at the lower temperatures found in the convective sections ofthe boiler.

Low-Light Imaging

In regions of the process where the temperatures are even cooler, thecamera can be used effectively by switching to a low f/# lens whichcollects more light and projects a brighter image onto the imagedetector inside the camera. Specially designed f/1 and f/0.9 lenses havebeen used to view regions that operate at temperatures as low as 500° F.If the narrow band filter is not used, allowing the image detector torespond to light over a much wider band, the camera has sufficientsensitivity to produce images of room temperature objects, but the needto limit the sensitivity of the system to narrow windows between themolecular absorption bands makes it difficult to produce images ofprocess components at temperatures lower than 500° F.

In regions of the process, where the temperatures are low (about 600°F.) and there is very little contrast due to everything being at thesame temperature and coated with ash which makes the emissivity of allthe surfaces the same, better contrast can be attained by using anexternal light source to illuminate the scene and produce shadowing.

A portable inspection camera (lightweight, handheld with handle, batteryoperated,) can be used with a lens assembly whose objective assembly isinserted into the interior of a process chamber thru a port/opening pastthe interior of the wall which would otherwise limit its field of view.

Obviously, numerous modifications and variations of the preferredembodiment described above are possible and will become apparent tothose skilled in the art in light of this specification. For example,many functions and advantages are described for the preferredembodiment, but in some uses of the invention, not all of thesefunctions and advantages would be needed. Therefore, we contemplate theuse of the invention using fewer than the complete set of notedfunctions and advantages. Moreover, several species and embodiments ofthe invention are disclosed herein, but not all are specificallyclaimed, although all are covered by generic claims. Nevertheless, it isour intention that each and every one of these species and embodiments,and the equivalents thereof, be encompassed and protected within thescope of the following claims, and no dedication to the public isintended by virtue of the lack of claims specific to any individualspecies. Accordingly, we expressly intend that all these embodiments,species, modifications and variations, and the equivalents thereof, areto be considered within the spirit and scope of the invention as definedin the following claims, wherein we claim:

1. An apparatus for inspecting the interior of high temperature in-furnace process equipment, comprising: an imaging sensor having a spectral sensitivity that is responsive to infrared radiation in a wavelength band of 3.5 to 4.1 microns; a lens assembly containing high IR transmissivity optic elements for focusing an image formed by said infrared radiation on said imaging sensor, and means for limiting spectral response of said sensor to a wavelength band of 3.5 to 4.1 microns.
 2. An apparatus for inspecting the interior of high temperature process equipment, comprising: an imaging sensor having a spectral sensitivity that is responsive to infrared radiation in a wavelength band of about 3.5-4 microns; a lens assembly containing high IR transmissivity optic elements for focusing an image formed by said infrared radiation on said imaging sensor, and means for limiting spectral response of said sensor to a preferred wavelength band; said spectral response limiting means includes an optical filter that absorbs infrared radiation in wavelength regions other than those that are essentially free from interference due to molecular absorption and emission, including a wavelength band of about 3.5-4 microns, and absorbs infrared radiation that would cause interference due to scattering by suspended particles in said equipment interior.
 3. An apparatus as in claim 2 wherein: said spectral response limiting means includes spectral sensitivity limits of said image detector in either short wavelengths or long wavelengths.
 4. An apparatus as in claim 2, wherein: said spectral sensitivity limiting means has a wavelength limit produced by transmission characteristics of optical material used in said optic elements of the image forming optics.
 5. An apparatus as in claim 2 wherein: either the short wavelength limit or the long wavelength limit of the spectral sensitivity limiting means is produced by interference coatings applied directly to the lens elements or to protective windows in the optical lens assembly.
 6. An apparatus as in claim 5 wherein: said lens elements are made of infrared transmissive solid materials including crystalline silicon or germanium.
 7. A process for visually inspecting the interior of a process chamber operating at temperatures above 500 degrees F., comprising: inserting a lens tip of a light weight portable battery operated, hand held camera with a handle grip into a port/opening past an interior wall of the chamber; transmitting and focusing infrared radiation from said interior through a lens assembly containing high IR transmissivity optic elements, and focusing an image formed by said infrared radiation on an imaging sensor; limiting spectral response of said sensor to a preferred wavelength band in a wavelength band of about 3.5-4 microns that is essentially free from interference due to molecular absorption and emission.
 8. An apparatus for inspecting the interior of process equipment operating at temperatures as high as 2500 degrees F., comprising: an imaging sensor having a spectral sensitivity that is responsive to infrared radiation in a wavelength band of about 3.1-4.1 microns; a lens assembly containing high IR transmissivity optic elements for focusing an image formed by said infrared radiation on said imaging sensor, and means for limiting spectral response of said sensor to a preferred wavelength band, wherein said preferred wavelength band is given in the following table for the listed process equipment: Black Liquor Recovery Boiler 3.36 microns to 4.15 microns Coal-fired Boiler 3.12 microns to 4.15 microns Fuel Oil-Fired Boiler 3.09 microns to 4.15 microns Natural Gas-Fired Boiler 3.11 microns to 4.15 microns Coal Gasifier  3.48 microns to 4.15 microns. 